Well-defined tricobalt tetraoxide's critical morphology effect on the structure–reactivity relationship

This review focuses on exploring the intricate relationship between the catalyst particle size and shape on a nanoscale level and how it affects the performance of reactions. Drawing from decades of research, valuable insights have been gained. Intentionally shaping catalyst particles makes exposing a more significant percentage of reactive facets possible, enabling the control of overactive sites. In this study, the effectiveness of Co3O4 nanoparticles (NPs) with nanometric size as a catalyst is examined, with a particular emphasis on the coordination patterns between oxygen and cobalt atoms on the surface of these NPs. Investigating the correlation between the structure and reactivity of the exposed NPs reveals that the form of Co3O4 with nanometric size can be modified to tune its catalytic capabilities finely. Morphology-dependent nanocatalysis is often attributed to the advantageous exposure of reactive crystal facets accumulating numerous active sites. However, experimental evidences highlight the importance of considering the reorganization of NPs throughout their actions and the potential synergistic effects between nearby reactive and less-active aspects. Despite the significant role played by the atomic structure of Co3O4 NPs with nanometric size, limited attention has been given to this aspect due to challenges in high-resolution characterizations. To bridge this gap, this review strongly advocates for a comprehensive understanding of the relationship between the structure and reactivity through real-time observation of individual NPs during the operation. Proposed techniques enable the assessment of dimensions, configuration, and interfacial arrangement, along with the monitoring of structural alterations caused by fluctuating temperature and gaseous conditions. Integrating this live data with spectroscopic methods commonly employed in studying inactive catalysts holds the potential for an enhanced understanding of the fundamental active sites and the dynamic behavior exhibited in catalytic settings.


Introduction
A catalyst's principal role is to facilitate the acceleration of a given process without compromising the reaction's integrity.Pt-group metals such as Ir, Ru, Pt, and Pd are acknowledged for their exceptional catalytic activity. 1,2However, their high cost and scarcity on Earth have motivated scholars to investigate cost-effective alternatives with an abundance on the planet. 3,40][11][12] With its three unoccupied d orbitals, cobalt bonds with surface-bound chemical species, enhancing catalytic activity, especially structural aws such as vacancies near the crystal lattice surface. 13Cobalt-based catalysts including tricobalt tetraoxide (Co 3 O 4 ) have gained signicant attention in Europe for their extensive use in energy and environmental industries.The catalytic properties of Co, attributed to its partly lled d orbital (3d 7 ), allow for facile composite creation by combining it with other elements or supports.Co and Co-based nanostructures have been investigated to enhance the surface area of catalysts, therefore exposing a more signicant number of active sites and allowing the selective exposure of the most active catalytic centers.Co's ability to transition between the Co 2+ and Co 3+ oxidation states based on redox conditions makes it an ideal reagent complex builder. 14

Preparation strategy of the morphological Co 3 O 4
Much effort has been dedicated to preparing Co 3 O 4 with wellcontrolled shapes, sizes, and crystal structures.6][37][38][39][40] NPs enclose outstanding features, such as a simple and economical synthesis method, high surface area, good stability, and uncomplicated recovery.These properties put together more approval than other synthesis strategies of the prepared catalysts.Researchers have tried to prepare Co 3 O 4 with nanometric size by different shapes using different methods to obtain a cost-effective, simple procedure, shorter time through an effective manner, and rectify the purity of the synthesized prepared sample.These processes include.

Coprecipitation
The coprecipitation method is a simple, efficient, and economical method for the mass production of ultrane nanopowders.Homogeneity, purity, and reactivity of the prepared oxide are the other advantages of this method.This method was used to prepare Co 3 O 4 with nanometric size. 35First, Co(NO 3 ) 2 -$6H 2 O was dissolved in deionized water.Secondly, ammonium oxalate was added to the solution with continuous stirring.The precipitate was then washed with deionized water and dried at room temperature.Finally, it was calcined at 400-500 °C for 3 h.The average size of the obtained NPs was from 40 to 350 nm, and the Co 3 O 4 NPs have an average diameter of 100 nm.

Utrasonic spray pyrolysis
Ultrasonic spray pyrolysis is an efficient, controlled, and versatile synthesis method.It is frequently used to prepare transition metal oxides, particularly Co 3 O 4 , 36 through high purity and narrow size distribution.
Three different precursor solutions were prepared by dissolving cobalt acetate, cobalt chloride, or cobalt nitrate in distilled water with a concentration of cobalt salt as 0.5 mol L −1 . 36The starting solution was aerosolized using an ultrasonic nebulizer (Omron, model NB-150U) with a frequency of 1.75 MHz.The spray pyrolysis temperature was kept at 750 °C.The obtained powders were collected at the reactor exit.

Thermal decomposition
The thermal decomposition of metal oxides performed in high boiling point organic solvents and the existence of surfactants are highly relevant.2][43][44][45] Nonetheless, this approach has some associated drawbacks, e.g., it requires preparation at high reaction temperatures, an inert atmosphere, and long processing times, resulting in increased energy and time consumption.
For example, cobalt oxalate was used as a precursor for synthesizing Co 3 O 4 NRs by thermal decomposition. 370.6 g of cobalt oxalate and 5 mL oleylamine (as a surfactant) were placed in a 50 mL two-neck distillation ask and heated up to 140 °C for 1 h.The resulting solution was added to 5 g of triphenylphosphine (as a surfactant) at 240 °C.The black solution was maintained under stirring at 240 °C for 45 min and then cooled to room temperature.The nal sample was washed with ethanol several times to remove the excessive surfactant.Transmission electron microscopy (TEM) was used to verify the size and shape of the prepared samples.The TEM images of Co 3 O 4 NRs demonstrated that the materials had rod-like shapes.The length of NRs was 400-550 nm, and their diameters were about 20 nm.

Microwave-assisted methods
Microwave-assisted chemistry is becoming essential in every area of synthetic chemistry since it can boost some competitive advantages over other preparation methods.It could reduce the processing times and enhance the crystallization level of the particles.These advantages of microwave-hydrothermal methods over conventional hydrothermal methods arise from the direct interaction of the microwaves with the ions or molecules in the solution and with the solid phases dispersed in the liquid medium.In effect, it is essential to underline that the efficiency in the conversion capacity of microwave energy into thermal energy is governed by the physics variables: loss tangent, relaxation time, and penetration depth. 46Non-aqueous solvents (glycerol, ethylene glycol (EG), propylene glycol) have been frequently used 47,48 to avoid or minimize the agglomeration process between the particles.
This method produces high yields, simple to operate, and efficient in terms of being environmentally friendly and energyconsuming.Also, it has been extensively applied to prepare inorganic nanostructured materials [49][50][51][52][53][54][55][56] with applications, e.g., electrodes, 57 humidity sensors, 58 or catalytic devices. 52The method's versatility for synthesizing NPs has been especially reported. 59The microwave-assisted hydrothermal route has been developed to prepare Co 3 O 4 with NRs' shape. 40The method involved two steps: rst, NRs of cobalt hydroxide carbonate were prepared by mixing 50 mL of 0.6 M Co(NO 3 ) 2 $6H 2 O and 2.4 g of CO(NH 2 ) 2 under 500 W microwave irradiated for 3 min.Subsequently, the cobalt hydroxide carbonate NRs were calcined under air at 400 °C for 3 h to fabricate Co 3 O 4 NRs.Aer the thermal decomposition of cobalt hydroxide carbonate precursor under 400 °C for three hours, a single phase of well-crystallized Co 3 O 4 with the cubic structure was obtained, and no peaks of the other phase were detected, indicating that the sample was of high purity.The as-prepared sample was bamboo-like NRs with a diameter varying from 30 to 60 nm and a length of 100 to 1000 nm.

Hydrothermal and solvothermal methods
The hydrothermal method is one of the best-used processes for preparing nanomaterials.It is essentially a solution reaction-based approach.To control the shape of the prepared materials, either low-pressure or high-pressure conditions can be used depending on the vapor pressure of the main composition in the reaction.It has numerous advantages over the other conventional methods such as energy saving, simplicity, costeffectiveness, acceleration interaction between solid and species, better nucleation control, higher dispersion, pollutionfree (as the reaction is done in a closed system), higher rate of the reaction, and lower temperature of operation in the presence of a suitable solvent.Also, it provides highly crystalline particles with better control over their size and shape.
The solvothermal process is similar in its technology to the hydrothermal one, as it is carried out in autoclaves at high temperatures and pressure, through just one difference: instead of water, the synthesis is carried out in organic solvents.Co 3 O 4 nanostructures with different morphologies (NCs, nanowires, nanobundles, nanoplates (NLs), and nanoowers) have been prepared, 38,52 and the experimental details of the preparation of Co 3 O 4 nanostructures with different shapes are summarized in Table 1.

Tuning the morphology of Co 3 O 4 for the catalytic reaction
Cobalt oxide is used mainly as a catalyst.Co 3 O 4 was used as a model in oxidizing CO. [60][61][62][63][64][65] An initial investigation revealed that Co 3 O 4 might facilitate the oxidation of CO at temperatures as low as −54 °C.The activity was signicantly decreased, however, when the reaction gas included trace amounts of moisture (3-10 ppm), which obscured the active Co 3+ sites. 66,67obalt oxide's activity and durability in the CO oxidation process were increased by changing its form from spherical NPs to NRs, demonstrating a solid morphology-dependent impact. 64R-shaped cobalt hydroxycarbonate was generated by precipitating cobalt acetate with sodium carbonate in EG.As seen in Fig. 1a-c, further calcination at 450 °C in air converted this precursor into rod-shaped Co 3 O 4 NR measuring 200-300 nm in length and 10-20 nm in diameter.The CO oxidation method using spherical NPs yielded an initial CO conversion of 30% at −77 °C.However, as the time-on-stream increased, this conversion decreased to around 10% (Fig. 1h).More active and stable than Co 3 O 4 NP catalysts, NR catalysts demonstrated 100% CO conversion in the rst 6 h and maintained an 80% CO conversion for ∼12 h aer the reaction.
In contrast to the spherical NPs, Co 3 O 4 NR demonstrated an approximately one-order-of-magnitude increase in the rate of CO oxidation.At −77 °C, the Co 3 O 4 NR reaction rate was 3.91 ×    The performance of Co 3 O 4 nanobelts (NBs) and NCs in CO oxidation has been investigated. 63The reaction rate of Co 3 O 4 NC, which mostly exposed the ∼{001} facets, was 0.62 mmol g −1 s −1 , as opposed to the 0.85 mmol g −1 s −1 seen on NBs terminated by the {110} plane.The specic conversion rate indicates that at 56 °C, Co 3 O 4 NB exhibits 1.37 times the activity of CO 3 O 4 NCs, demonstrating that the Co 3 O 4 NB are signicantly more active than Co 3 O 4 NC.As shown by these studies, the activation of the surface layer lattice oxygen on the {110} planes is more pronounced in the presence of Co 3+ species compared to the     same planes had superior activity at a reduced activation energy of 21 kJ mol −1 .Signicant morphology-dependent effects on CO oxidation have been observed, contradicting prior hypotheses to some degree (maybe due to the porous structures amid cracks and interspaces in the Co 3 O 4 nanostructures).The formation of Co 3 O 4 NS, Co 3 O 4 NB, and Co 3 O 4 NC by hydrothermal synthesis of a cobalt hydroxide precursor followed by direct thermal breakdown was investigated in kinetic experiments for methane (CH 4 ) combustion (Fig. 2a-f). 30The specic rates (r CH4 ) for Co Beyond these crystal planes, the methane combustion process persisted in the following order: {112} > {110} J {001}.It can be deduced that manipulating the structure of nanostructured cobalt oxides leads to a substantial display of catalytically active sites.This is supported by the enhanced CH 4 combustion activity observed in Co 3 O 4 as a nanosheet, which exposes the more reactive {112} planes.The catalytic activity of Co 3 O 4 supported on stainless steel wire mesh, produced by the ammonia evaporation process, was investigated with the preferred oxidation (PROX) of CO. 68 The 500 nm-diameter mesoporous Co 3 O 4 nanowires' diameter is 3.4 nm, and they have a Brunauer-Emmett-Teller (BET) surface area of 71 m 2 g −1 .This structured catalytic system is very stable over the whole temperature range of 100-175 °C due to its low-pressure drop and high heat exchange rate; furthermore, its exceptional catalytic activity is twice that of the highest-performing Co 3 O 4 catalyst previously documented.
Although PROX was believed to have an active Co 3+ site, its mechanism may have been distinct from the low-temperature oxidation of the CO reaction.Researchers   71 due to its plentiful Co 2+ and more reactive surface, in addition to its most excellent surface area (121.1 m 2 g −1 ).The core-shell contrast ratio of the as-prepared Co 3 S 4 @Co 3 O 4 core-shell octahedron catalyst via hydrothermal and post-surface lattice anion exchange is comparatively less than that of the other core-shell structures. 72This is because the concentrations of Co 3 S 4 and Co 3 O 4 are close.The hexagonal shape of the selected area electron diffraction pattern, as seen in Fig. 5E, corresponds to both the {111} facet exposure and the close-packed hexagonal pattern observed in the inset of Fig. 5D in HRTEM.The lattice spacing of the {220} pattern is 0.33 nm.As seen in Fig. 5G, electrochemical CO 2 reduction reaction (CRR) and oxygen reduction reaction (ORR) were investigated using a core-shell conguration of Co 3 O 4 NO coated with a Co 3 S 4 surface.A distinctive electronic conguration is bestowed by the heterojunction separating the p-type Co 3 O 4 core and the n-type Co 3 S 4 shell, enabling both catalytic processes.
To solve the recovery issue and make a reusable, eco-friendly "green" catalyst, the optimum catalyst is Co 3 O 4 with nanometric size attached to a particular substrate with solid adhesion.Chemical (sol-gel), physical (pulsed laser deposition, or PLD), and electrochemical (electroless) methods have been used to create coatings that are reconstructed with Co 3 O 4 with nanometric size.Fig. 6a and b shows that the Co 3 O 4 NPs generated using the PLD approach without post-annealing treatment have a mixed amorphous-nanocrystalline phase, a tiny average size of 18 nm, a narrow size distribution of s = 3 nm, a perfectly spherical form, and allow a degree of accumulation. 73,74n a methylene blue (MB) solution, the activity of a homogeneous catalyst generating Co 2+ ions was compared to that of a thin coating catalyst constructed with heterogeneous Co 3 O 4 with nanometric size.Complete mineralization of MB dye was achieved in 240 min, indicating a far greater degradation rate than the 40% removed by Co 2+ ions (Fig. 6c).In the same study, researchers 73 found that coatings made of assembled Co 3 O 4 with nanometric size had a slightly lower catalytic activity but still demonstrated good recycling capability.Despite applying the same quantity of Pd NPs to these materials, the Pd/Co 3 O 4 NS catalyst continued to produce the most methane combustion.The PdO {111} and Co 3 O 4 NS formed a geometrically advantageous match, particularly on the {112} facet (Fig. 7a-c), which enhanced the solid metal support interactions and subsequently facilitated the activation of C-H bonds. 75The number of missing neighbors of a Co 3 O 4 unit cell on a plane {112} is ve for the NS shape.PdO must be sited in the 5-fold center of the surface of Co 3 O 4 NS as a thin discrete lm through a matching geometry and strong coordination rather than a top or bridge site. 76ue to its low activation barrier of 29.6 kJ mol −1 , single Pt atoms attached to Co 3 O 4 demonstrate signicant catalytic activity in the water gas shi process at 200 °C (turnover frequency = 0.58 mol H2 per site Pt per s).The signicantly decreased activation energy observed for these individual Pt Furthermore, the catalysts Au/Co 3 O 4 P were evaluated in the CO oxidation processes. 80The catalytic activity was substantially enhanced by adding Au NPs, as shown in Fig. 9h.This resulted in a noteworthy CO conversion of 35% at 20 °C and complete at 80 °C.As depicted in Fig. 9i, the activation energy (E a ) for CO oxidation in Au/Co 3 O 4 P is 15.49 kJ mol −1 .Therefore, oxygen molecules follow the Langmuir-Hinshelwood mechanism, which catalyzes CO oxidation at low temperatures (20-60 °C) via Au/Co 3 O 4 P {111}.In particular, rather than traversing the surface lattice oxygen sites, CO should be adsorbed onto oxygen vacancies at the surface and activated by Au NPs.The durability of the Au/Co 3 O 4 P catalysts was also evaluated at temperatures of 25 and 60 °C (Fig. 9j).Throughout the twelve-hours CO oxidation process at 25 °C, the Au/Co 3 O 4 P catalyst activity decreased from 2.92 to 1.87 mol reactedCO g Au −1 s −1 .A minimum activity of 5.26-5.39mol reactedCO g Au −1 s −1 was recorded for 9 h at 60 °C.This phenomenon might be primarily attributed to the surface oxygen vacancies and inherent defects of Co 3 O 4 {111}, which activated O 2 .Similarly, the presence of Au 0 , Au d+ , and Au + species on the surface of Au NPs further enhanced the activation of CO.

Chemical nature of the oxide particle morphology
Many people think that certain cobalt cations are abundant at the active sites.However, these ndings were mainly obtained via catalytic research, and direct spectroscopic evidence of the active surface oxidation state was absent.Contrary to comparable nanostructures, there have been consistent ndings on the shape inuence of Co 3 O 4 with nanometric size in catalyzing oxidation processes (as shown above).The many reaction routes can contribute, including changing the reaction conditions (primarily the gas and temperature).CO may be oxidized by the Langmuir-Hinshelwood method, which requires surface oxygen species, or the Mars-van Krevelen mechanism, which utilizes lattice oxygen species, according to spectroscopic observations 82 and the spectroscopically examined possible reaction pathways/ elementary steps of CO oxidation on Co 3 O 4 are congured in Fig. 10, the former exhibited dominance at over 100 °C due to oxygen vacancy formation and the Co 3+ /Co 2+ redox cycle.Conversely, at lower temperatures, the latter demonstrated dominance.One possible reaction mechanism is that CO adsorbs onto Co 3+ cations and then absorbs oxygen from the surface lattice coordinated to three Co 3+ cations.The oxygen vacancy is then lled with oxygen from the gas phase, according to the Mars-van Krevelen mechanism. 64pectroscopic evidence is lacking, although an interaction between molecularly adsorbed CO and O-O peroxo species has been postulated by analyzing the impact of pretreatment conditions, 65 although no peroxo O-O species were found using in situ Raman spectroscopy. 81According to in situ infrared research, CO adsorbed on Co 2+ sites interacted with an oxygen atom bound to a nearby Co 3+ cation, and the gas phase oxygen was used to ll the oxygen vacancy. 83Isotopes are vital in the redox Mars-van Krevelen process and are responsible for CO oxidation. 84,857][88][89] For instance, a Marsvan Krevelen process involving mostly exposed {110} planes in Co 3 O 4 has been proposed, as shown in Fig. 11. 88heoretically, the octahedrally coordinated Co 2+ site in CoO 90 would be the most active site for the PROX of CO in the hydrogen-rich stream.According to DFT calculations, the generated carbonates should make the {001} facet of Co 3 O 4 less reactive by blocking the surface sites on that facet but not on CoO {001}, as shown in Fig. 12.
Surface and lattice oxygen species interact concurrently in the reaction network, making methane oxidation on Co 3 O 4 catalysts more difficult.There were three distinct temperature/ conversion phases in the methane oxidation process, identi-ed by the presence or absence of the adsorbed or lattice oxygen and the catalyst's redox state. 91At temperatures between 300 and 450 °C, the dominating supercial Langmuir-Hinshelwood structure produces a stoichiometric {100} surface on Co 3 O 4 NC   97 The surface remodeling during reaction circumstances may contribute to the contradicting ndings on the reactive facets.It has been shown by molecular modeling of Co 3 O 4 NPs that the form may be maintained; however, when exposed to oxidizing and reducing atmospheres, the relative ratio of {111}/{100}/{110} facets changes dynamically. 98Under conditions rich in hydrogen gas, the faceting {110} plane was preferentially exposed.At the same time, the {111} surface remained untreated due to the development of oxygen surface vacancies and their subsequent diffusion toward the bulk.Nevertheless, the oxygen-rich conditions promoted the {111} termination.Therefore, it was necessary to describe the shape of the active catalysts.Recent breakthroughs in highresolution microscopic and spectroscopic methods have opened the door to studying the functions of shapedsynchronized NPs in terms of their dynamic performance.Nitric oxide (NO) may be reduced with CO by reshaping Co 3 O 4 NRs with an exposed {110} surface into non-stoichiometric CoO 1−x NR (Fig. 13a and b). 99The structure-modied NRs generate nitrogen gas by selectively reducing nitrogen oxides (NO x ) with CO at temperatures ranging from 250 to 520 °C.Environmental transmission electron microscopy (ETEM) and ambient pressure X-ray photoelectron spectroscopy showed that the non-stoichiometric CoO 1−x NRs had a rock-salt (RS) structure.The 100% selectivity was brought about by the active phase, which included around 25% oxygen vacancies.Electron transport microscopy measurements in environments rich in hydrogen showed that CO 3 was reduced to CO, indicating the formation of a boundary contact for particles larger than 15 nm but not for smaller ones, showing that smaller NPs undergo rapid reduction. 100ETEM identied a two-step phase transition during the heating experiment, as shown in Fig. 13c and d

Concluding remarks and perspectives
Extensive exploration into the eld of nanocatalysis utilizing Co 3 O 4 nanometrics has undeniably demonstrated that the size and shape of the catalyst at the nanoscale level profoundly impact its catalytic effectiveness.A growing body of evidence suggests that the conguration of the nanometric Co 3 O 4 is always critical in achieving optimal levels of selectivity, stability, and catalytic activity.This technology's advancement has been signicant due to the incorporation of morphology-dependent nanocatalysts, an innovative tool for nely adjusting catalytically active sites.Both theoretical and experimental investigations have been extensive into the morphology-dependent nanocatalysis of nanometric Co 3 O 4 .Specically, the arrangement of surface Co 3+ /Co 2+ and O sites, [102][103][104] focusing on the oxygen vacancy, has been linked to the catalytic properties of reactive surface facets.However, there are conicting reports regarding the effectiveness of similar nanostructures in catalyzing different processes or even the same reaction under identical conditions.This suggests that the form-dependency of nanometric Co 3 O 4 , as documented, is highly susceptible to variations in reaction parameters and established reaction pathways.
The relationship between the catalytic activities of nanometric Co 3 O 4 and the selectively exposed facets induced by shape has been demonstrated through experimental evidence.However, it cannot be ruled out that adjacent facets may work together synergistically.Initially designed nanostructures may undergo structure, morphology, and chemistry changes under actual reaction conditions.The catalytic properties observed in the experiments are determined by the dynamic behavior of the catalyst particles in response to temperature and the reactive environment rather than their state when prepared or recently used.In some instances, the activation of species in a multimolecule chemical reaction may occur through diffusion on adjacent facets, resulting in a synergistic effect where the species activated by the adsorbed reactant can adsorb and stimulate a different type of reactant.In situ studies, physical and chemical analyses, and dynamic characterization techniques must be employed in operational environments to fully understand functional nanostructures.
][107] Variations in temperature and reactive gas uctuations can impact the well-dened form of Co 3 O 4 nanometric, leading to changes in its electrical and geometric properties.This, in turn, inuences the proportion of active surfaces and the coordination environments of oxygen and cobalt atoms on the surface, ultimately affecting the development of active sites.The lack of published studies on the atomic structure of nanometric Co 3 O 4 can be attributed to the limited availability of high-resolution spectroscopic and microscopic characterizations among researchers worldwide.Also, studying active sites' dynamic performance under operational conditions would provide valuable insights into the structure-reactivity relationship.By employing techniques that allow for real-time assessment of size, shape, interfacial structure, and gas-induced structural changes at the active sites of individual nanoparticles, combined with spectroscopic methods, we can signicantly enhance our understanding of the inherent active regions and dynamic capabilities of nanostructured catalysts within catalytic environments.
The prepared Co 3 O 4 samples from cobalt acetate, cobalt chloride, and cobalt nitrate are denoted as A-Co 3 O 4 , C-Co 3 O 4 , and N-Co 3 O 4 .According to the X-ray diffraction (XRD) data of A-Co 3 O 4 , C-Co 3 O 4 , and N-Co 3 O 4 samples, all the prepared samples adopted a spinel-type cubic structure.The characteristic diffraction peaks are sharp, and no impurities or a second phase were detected, affirming that high-purity Co 3 O 4 was obtained.Scanning electron microscopy (SEM) was used to examine the shapes of the A-Co 3 O 4 , C-Co 3 O 4 , and N-Co 3 O 4 samples.For the A-Co 3 O 4 powders, the dimple and wrinkle surface can be observed.C-Co 3 O 4 sample has a porous spherical morphology, and microspheres are developed from various closely packed primary particles; moreover, abundant voids are le among adjacent particles.The N-Co 3 O 4 sample has a durian-like shape with a 0.5-3 mm size distribution, suggesting a hollow inner structure.

10 − 6
mol CO g −1 s −1 .Conversely, the value of the NPs was just 4.66 × 10 −7 mol CO g −1 s −1 .The high-resolution transmission electron microscope (HRTEM) analysis revealed that the Co 3 O 4 NPs were enclosed by a conguration consisting of eight {111} and six {001} planes.Conversely, the Co 3 O 4 NR preferred to reveal the {110} planes, constituting an estimated 40% of their overall surface area (Fig.1g).It was found that Co 3+ species functioned as active sites for CO oxidation on the {110} plane.

Fig. 2
Fig. 2 Scanning electron microscopy (SEM) and high-resolution transmission electron microscope (HRTEM) analysis with structural models of Co 3 O 4 nanostructures: (a and b) Co 3 O 4 NS; (c and d) Co 3 O 4 NB; (e and f) Co 3 O 4 NC.(g) Methane conversion efficiency vs. temperature for Co 3 O 4 at a GHSV of 40 000 h −1 (based on ref. 30, Copyright 2008, American Chemical Society).
{001} planes.Furthermore, it was shown that Co 3 O 4 nanowires (NWs) enclosed in {111} planes and measuring around 3 nm in diameter had a notably increased rate of CO oxidation at 248 °C, amounting to 161.75 mmol CO g −1 s −1 .61The enhanced performance resulted from the increased surface area and the profusion of Co 3+ cations on the surfaces.A catalytic study for CO oxidation 62 indicates that Co 3 O 4 NR exposed to {111} planes exhibited enhanced activity at an activation energy of 40 kJ mol −1 , whereas Co 3 O 4 NLs exposed to the

Fig. 6
Fig. 6 Co 3 O 4 catalysts synthesized via various methods and their photocatalytic performance: (a) SEM images of coatings prepared by PLD, (b) particle size distribution histogram for PLD coatings, (c) time-dependent photocatalytic degradation of MB using Co 3 O 4 NPs assembled coating via PLD and cobalt nitrate and (d-g) SEM images respectively of coatings prepared by (d) electroless, (e) electron beam, (f) sol-gel depositions, and powder form; (h) comparative photocatalytic efficiency of powder Co 3 O 4 and coatings by different methods (adapted from ref. 73 and 74 © 2012 Elsevier BV).
and surface characteristics, including shape, surface area, and facets, signicantly affect the catalytic activity.As seen in Fig.3a-l,16 the synthesis of Co 3 O 4 , including a variety of Co 3 O 4 NR {110}, Co 3 O 4 NC {100}, and nano-octahedron {111} (NO) facets has been completed.The catalytic reactivity of Co 3 O 4 NR, Co 3 O 4 NC, and Co 3 O 4 NO was the highest for phenol oxidation by the persulfate (PS) process.Fig. 3m and n demonstrated that the Co 3 O 4 NR exhibited the lowest adsorption energy estimated by the density functional theory (DFT).This conrms that PS is more easily activated via a non-radical pathway on the Co 3 O 4 {110} plane. 16To degrade 5-sulfosalicylic acid, four distinct 3D Co 3 O 4 catalysts were fabricated, each with a unique morphology (Fig. 4): Co 3 O 4 NC {111}, Co 3 O 4 NLs {110}, Co 3 O 4 NNs (nanoneedles, {110}), and Co 3 O 4 NFs (nanoowers, {112}). 71Primarily, Co 3 O 4 NF ({112} facets) is the most benecial 3D Co 3 O 4 catalyst for the oxidation activation to degrade 5-sulfosalicylic acid

Co 3 O
4 has been considered an active support for heterogeneous catalysis for a very long time and is distinguished by its solid metal support interactions.As stated, Co 3 O 4 with nanometric size has an evident morphological inuence on CH 4 combustion in the following sequence: Co 3 O 4 NS, Co 3 O 4 NB, and Co 3 O 4 NC.

Fig. 9
Fig. 9 Scanning transmission electron microscopy (STEM) imaging and catalytic performance of Au-doped Co 3 O 4 : (a-c) Au particles on Co 3 O 4 {001} in Au/Co 3 O 4 NC; (d-f) Au particles on Co 3 O 4 {111} in Au/Co 3 O 4 nanoparticles (NPs); (g) catalytic efficiency of Au/Co 3 O 4 NPs and Au/ Co 3 O 4 NC in ethylene glycol (EG) oxidation.Reprinted with Permission from ref. 79, 2021 Royal Society of Chemistry.Performance analysis of Co 3 O 4 and Au/Co 3 O 4 in CO oxidation: (h) temperature-dependent catalytic activity for CO oxidation; (i) Arrhenius plots showing rate vs. 1/T for CO oxidation over Au/Co 3 O 4 ; (j) durability tests at 25 °C and 60 °C with CO conversion rates from 25% to 45%.Reprinted with Permission from ref. 80, 2023 Royal Society of Chemistry.

Fig. 10
Fig. 10 Schematic representation of CO oxidation on Co 3 O 4 .Reprinted with Permission from ref. 82, 2018 American Chemical Society.
Co 3 O 4 NR, rich in Co 3+ cations and having mostly exposed {110} surfaces, is very active in low-temperature CO oxidation. 64Moreover, among Co 3 O 4 NR, Co 3 O 4 NC, and Co 3 O 4 NP, Co 3 O 4 NS with mostly exposed {111} planes enriched in Co 2+ cations are the most active. 38At low temperatures, a Co 3 O 4 SiO 2 nanocomposite devoid of ordered planes but abundant in Co 2+ proved an exceptionally active catalyst. 81

Fig. 11
Fig. 11 Three adsorption configurations of CO on Co 3 O 4 (110): (a) on O 2f ; (b) on O 3f ; (c) on Co.Bond lengths are in angstroms; bond angle are in degrees.Co, green, O, blue, and C, red.Reprinted with Permission from ref. 88, 2011 Royal Society of Chemistry.

Fig. 12
Fig. 12 Potential energy diagrams for (a) the hydrogenation of Co 3 O 4 {001} and (b) the oxidation of CO to CO 2 on Co 3 O 4 {001} and CoO {001}.For each transition state (hollow boxes), reaction barriers are given in kJ mol −1 .Selected intermediates are shown as a side view along [110], using the following color codes: black (C), blue (Co), red (O), and white (H).Reprinted with Permission from ref. 90, 2019 American Chemical Society.
. In the low-temperature range of 200 to 280 °C, the wurtzite (WZ) CoO was spontaneously oxidized to spinel (SP) Co 3 O 4 owing to the residual oxygen in the TEM.Secondly, under low oxygen partial pressure conditions, SP Co 3 O 4 was reduced to RS CoO at temperatures reaching 280 °C. 101These visual results show that the as-prepared oxide NPs changed signicantly under response conditions.

Fig. 13
Fig. 13 Structural transformation of Co 3 O 4 NR: (a) high-resolution transmission electron microscope (HRTEM) image; (b) schematic illustration of Co 3 O 4 to CoO transformation under reaction conditions; (c) HRTEM image of CoO hexagonal pyramid; (d) illustration of the phase transformation from metastable wurtzite (WZ) CoO to stable rock-salt (RS) CoO via the intermediate spinel (SP) Co 3 O 4 .Reprinted with Permission from ref. 90 and 101 Copyrights 2013 and 2019, American Chemical Society.

Table 1
Experimental parameters of the preparation of different shapes of Co 3 O 4 nanostructures